原子气体中的恒星形成
Star Formation in Atomic Gas
摘要：
对附近星系的观测已经在广泛的星系环境和金属丰度范围内确定了恒星的形成只发生在星际介质的分子阶段。理论模型表明，这种关联的结果从化学相，屏蔽和温度之间的相关性。星际气体只有在受到星际紫外线（UV）光子屏蔽的区域才能从原子转变为分子，由于UV光子也是星际加热的主要来源，只有在这些屏蔽区域，气体才会变得足够冷，从而受到金斯不稳定性的影响。然而，虽然星际气体的平衡温度和化学状态是密切相关的，但达到化学平衡所需的时间尺度要比达到热平衡所需的时间尺度长得多，而且这两种时间尺度都与金属度有关。在这里，我表明，时间尺度的差异意味着，在金属丰度低于太阳值的百分之几时，屏蔽良好的气体将达到低温，并在大部分气体能够从原子转化为分子之前进行恒星形成。因此，在极低的金属丰度下，恒星的形成将发生在ISM的冷原子阶段，而不是分子阶段。我计算的结果，在低金属量星系的恒星形成的可观察到的后果，我讨论了目前的一些数值模型H2调节恒星形成可能需要修改。
英文摘要：
Observations of nearby galaxies have firmly established, over a broad range of galactic environments and metallicities, that star formation occurs exclusively in the molecular phase of the interstellar medium (ISM). Theoretical models show that this association results from the correlation between chemical phase, shielding, and temperature. Interstellar gas converts from atomic to molecular only in regions that are well shielded from interstellar ultraviolet (UV) photons, and since UV photons are also the dominant source of interstellar heating, only in these shielded regions does the gas become cold enough to be subject to Jeans instability. However, while the equilibrium temperature and chemical state of interstellar gas are well-correlated, the time scale required to reach chemical equilibrium is much longer than that required to reach thermal equilibrium, and both timescales are metallicity-dependent. Here I show that the difference in time scales implies that, at metallicities below a few percent of the Solar value, well-shielded gas will reach low temperatures and proceed to star formation before the bulk of it is able to convert from atomic to molecular. As a result, at extremely low metallicities, star formation will occur in a cold atomic phase of the ISM rather than a molecular phase. I calculate the observable consequences of this result for star formation in low metallicity galaxies, and I discuss how some current numerical models for H2-regulated star-formation may need to be modified.
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